Different types of devices are available to transport ions from one location to another, these devices being adapted to the pressure conditions of the surroundings. For transport in the vacuum there are several satisfactory solutions, including solutions which allow the ions to be focused to a beam in the axis of the transport system. For targeted, concentrated transport of ions in air or gases at atmospheric pressure, however, the only options are transport with the flowing gas or the phenomenon of ion mobility, by means of which ions drift through the gas along electric lines of force, being constantly decelerated by the gas. Neither type of transport can achieve a narrow focusing of the ions. Especially for targeted transport of ions located in a relatively large cloud in ambient air, no transport system with sufficiently low losses for transporting the ions into the vacuum of a mass spectrometer has yet been found.
In a very good high vacuum, ions can be transported in ion guides comprising an external tube and a thin wire mounted in the axis. A potential difference between wire and tube creates a field arrangement in which ions can be transported in the tube along the axis, the ions executing Kepler type motions around the wire beside their forward drift.
This type of ion guide cannot be used in a poorer vacuum, in which a moderate number of collisions with molecules of residual gas damp the motion of the ions, since the damped ions would be eventually discharged on the central wire. However, ion guides based on RF multipole rod arrangements according to Wolfgang Paul can be used very successfully here, since these form electric RF fields which accelerate the ions toward the axis of the rod arrangement. The damping of the oscillations transverse to the axis causes them to be collected eventually in the axis of the rod system. The ions can then be transported by residual gas in motion, or by space charge effects where, for example, the ions are removed at one end of the rod system by suction and are pushed on by the space charge effect.
Besides these rod systems, other ion guides have been described which can be operated with RF and additionally supplied with an axial potential difference, for example systems consisting of a large number of ring diaphragms arranged in parallel, or double helix systems (U.S. Pat. No. 5,572,035; J. Franzen). The axial potential difference guides the ions actively through the ion guide. A recently invented arrangement of parallel diaphragms with apertures of a very special shape makes it possible to collect the ions in the axis as well as actively transporting them forward (U.S. application Ser. No. 11/243,440; GB Application 0520291.6; J. Franzen et al.). A further variety of an ion guide is the ion funnel (U.S. Pat. No. 6,107,628; R. D. Smith and S. A. Shaffer), which can collect ions at pressures below one kilopascal from a relatively large cloud, free them to a large extent from the gas following behind and focus them. It consists of ring diaphragms whose apertures have tapering inside diameters and an axial potential difference.
Ions can survive for any length of time in air or other gases if the energy for ionizing them is greater than the energy for ionizing the ambient gases, and if neither ions of the opposite polarity nor electrons are available for recombinations. Ions can be transported through gases using electric fields, in which case the laws of ion mobility apply, according to which the ions move along the electric lines of force, being continuously decelerated and their direction being only slightly affected by diffusion motion.
The ions can also be transported by the moving ambient gas itself, however. If gas is forced through a tube or capillary, ions are viscously entrained in the gas. It is thus known that ions generated outside the vacuum system can be guided through a capillary into the vacuum of a mass spectrometer. When the ions are being transported through capillaries, however, they must be prevented from colliding with the wall, since these wall collisions generally discharge the ions and hence destroy them.
It is known from capillary chromatography that all the molecules of a gas that moves through a capillary suffer an extraordinarily high number of wall collisions. The number of wall collisions essentially corresponds to the number of the theoretical (vaporization) plates which represent the separation efficiency of chromatographic columns. In capillary columns it is extremely high. A rough rule of thumb for the best possible gas speed (the “van Deemter” speed) is that a molecule statistically collides once with the wall after a path which corresponds to the diameter of the capillary. For higher gas speeds, the number of wall collisions per unit of path length decreases. The wall collisions, however, are not evenly distributed along the capillary. Time and again there are long paths with no wall collisions, alternating with paths with much more frequent wall collisions. It follows that only those ions which happen to cover a long path without coming into contact with the wall can get through a capillary undamaged. It may be assumed that these ions have entered the capillary centrally.
The phenomenon of ion transport in capillaries was investigated in the paper “Ion Transport by Viscous Gas Flow through Capillaries” by B. Lin and J. Sunner in J. Amer. Soc. Mass Spectr. 5, 873 (1994). In this paper, the authors initially refuted the widely held view that the ions can be focused by applying a charge to the capillary walls. Inside a capillary with uniformly charged walls there is a field-free drift region in which ions cannot be focused at all. There is no repulsion of the ions whatsoever when they approach the charged wall. The authors' experiments showed that the diffusion of the ions toward the walls does actually cause high losses to an extent which was theoretically to be expected, and that only a statistically expected residual number of the ions can pass undamaged through the capillary. The yield of transported ions decreases with the length of the capillary, and there is a similar drastic reduction for thinner capillaries. A further loss occurs especially because of space charge effects, whose Coulomb repulsion drives the ions to the capillary walls. The space charge effect limits the transport of ions through such single capillaries.
It is also known that it is even possible to pump the ions against a potential difference by viscous entrainment of the ions in the gas stream, as described in the article “Electrospray Interface for Liquid Chromatographs and Mass Spectrometers” by C. Whitehouse et al., Anal. Chem. 1985, 57, 675. This is already used in commercially available instruments. This can be used to pump the ions up to an acceleration potential inside a mass spectrometer, for example; or the needle of an electrospray unit can be set to ground potential for safety reasons, and the inlet of the capillary can be set to spray potential.
In patent DE 195 15 271 C2 (J. Franzen, which corresponds to GB 2 300 295 B, U.S. Pat. No. 5,736,740 A) a gas-dynamic focusing is suggested, which has to occur when ions are transported against a potential difference in a capillary. The gas-dynamic focusing comprises a circulation lift of a molecule located close to the wall in the parabolic velocity profile of the gas flow and executes an ion mobility motion in the backward direction.
If a decelerated ion is not in the axis of the capillary, it experiences a slightly slower velocity of gas circulation on the side near the wall than on the side toward the central axis. Bernoulli's laws mean that this slight difference makes itself felt in a so-called circulation lift, which is directed toward the side of the higher gas speed, i.e., toward the axis. (The circulation lift of an aircraft wing, which keeps the aircraft in the air, is a well-known phenomenon, although generated in a slightly different way.) This gas-dynamic focusing power opposes the random diffusion motion of an ion toward the wall and brings the ion back to the axis of the capillary. The focusing power is proportional to the difference of the squares of the circulation speeds on both sides of the ion, and therefore increases, the greater the deceleration. It is not present when the ion moves at the speed of the ambient gas.
It has not yet proved possible to definitely detect this focusing effect as such, but the lower cut-off limit for ions of too low a mass-to-charge ratio associated with this effect has been detected. The focusing effect is expectably very small and very inferior to opposing space charge effects. The gas-dynamic focusing can therefore only be effective when no space charge effects whatsoever are present.
The paper “Improved Ion Transmission from Atmospheric Pressure to High Vacuum Using a Multicapillary Inlet and Electrodynamic Ion Funnel Interface” by T. Kim et al., Anal. Chem., 72, 5014-5019 (2000) describes how a bundle of seven identical metal capillaries can achieve much more than seven times the ion transport of a single metal capillary with the same dimension, soldered into the same kind of block, although the seven capillaries have to be equipped with a more powerful pump system in order to achieve roughly the same pressure in the ion funnel. How the bundle of seven capillaries achieves the 10- to 20-fold ion transport is as yet unexplained. Nor has it been explained how two different bundles whose individual capillaries have inside diameters of 0.51 and 0.43 millimeters respectively, and whose gas streams must differ mathematically by a factor of two, demonstrated a reduction of the ion transport of only 30 percent for the smaller diameter.
It can only be surmised that a mutual influencing of the gas streams means the inflow of the ions into the seven adjacent capillaries of the bundle is more organized than the inflow into a single capillary, and possibly leads to less turbulence in the inlet region of the capillary. That the organization of the gas at the capillary inlet is important is shown in the following paper: “Improved Capillary Inlet Tube Interface for Mass Spectrometry—Aerodynamic Effects to Improve Ion Transmission”, D. Prior et al., Computing and Information Sciences 1999 Annual Report. The authors report that a slightly funnel-shaped widening of the capillary inlet leads to a fourfold increase in ion transmission from an electrospray ion source.
With the prior art only a small proportion of the generated ions in an enclosed gas stream can be transported undamaged at a time.
The gas in the vacuum system of a mass spectrometer generally makes it necessary to have a differential pump system with at least three pressure stages. Commercially available electrospray instruments incorporate these pressure stages. In the first differential pump stage there is a relatively high pressure of around one to three hectopascal, which greatly impedes the onward transmission of the ions. The ions are usually accelerated toward skimmers located opposite the end surface of the capillaries. This causes high focusing and scattering losses. The use of ion funnels, as described above, improves the ion transport through this first pressure stage. In the second pressure stage it is then possible to capture the ions effectively, for example using an ion guide made of a multipole arrangement with long pole rods.